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Asset Retirement Obligation - Reconciliation of Total Reclamation Liability for Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Beginning balance   $ 13,322 $ 8,982
Additions and revisions of prior estimates $ 9,828 (5,114) 7,546
Accretion expenses   687 26
Settlement of liability   (2,753) (3,180)
Ending balance $ 6,142 $ 6,142 $ 13,322